THE HISTORY OF SOME DISCOVERIES OF PHOTOGRAPHY. 303 



Paper thus jjreparecl is exquisitely sensitive to lic:ht, an exposure 

 of less than a second to diffused daylight being quite sufficient to set 

 up the process of change. If a piece of this paper is partly covered 

 and the other exposed to daylight for tlie briefest possible period of 

 time, a very decided impression Avill be made. This impression is 

 latent and invisible. If, however, the paper be placed aside in the 

 dark, it will gradually develop itself; or it may be brought out im- 

 mediately by being washed over with the gallo-nitrate of silver, and 

 held at a short distance from the fire, by which the exposed portions 

 become brown, the covered parts remaining of their original color. 

 The pictures being thus procured, are to be fixed by washing in clean 

 water and lightly drying between l)l()tting paper, after which they 

 are to be washed over- with a solution of bromide of potassium con- 

 taining 100 grains of that salt dissolved in 8 or 10 ounces of water; 

 after a minute or two it is again to be dipped into w^ater, and then 

 finally dried. 



Such Avas. in all its main features, the description given by Mr. 

 1'albot in his specification of his process for producing the calotype, 

 or beautiful picture (as the term signifies). He, in a second patent, 

 included the points stated in the next section. 



SECTION HI. IMPRCWEMENTS IN CALOTYPE. 



Such is the term employed by Mr. Talbot, and these improvements 

 consist of the following particulars, constituting that gentleman's 

 second claim : 



1. Eemoviug the yellowish tint which is occasioned by the iodide 

 of silver from the paper by plunging it into a hot bath of hyposul- 

 phite of soda dissolved in ten times its weight of water and heated 

 nearly to the l)oiling ])oint. The picture should remain in the bath 

 about ten minutes and be then washed in warm water and dried. 



Although this has been included by Mr. Talbot in his specifica- 

 tion, he has clearly no claim to it, since in February, 1840, Sir John 

 Herschel published, in his memoir "On the chemical action of the 

 rays of the solar spectrum," a j)r()cess of fixing with the hot hyposul- 

 phite of soda. 



After undergoing the o})eration of fixing the ])icture is placed upon 

 a hot iron and wax melted into the pores of the })aper to increase 

 its transparency. 



2. The calotype paper is rendered more sensitive by i)la('ing a 

 Avarm ii'on behind in the camera while the light is acting upon it. 



3. The ])reparation of io-gallic paper, which is sim])ly washing a 

 sheet of iodized paper with gallic acid. In this state it will keep in 

 a portfolio, and is rendered sensitive to light by washing it over 

 with a solution of nitrate of silvei-. 



4. Iodized paper is washed with a mixture of 26 parts of saturated 



